With this adjustment device, you can accurately check your tribrach's optical plummet.

METHOD:
A tribrach is placed on a tripod under a ceiling. Insert the adjusting device into the tribrach, lock and level it. 

Then place the tribrach (with optical plummet to be tested) on the alignment device with the base plate facing upwards and lock it. 
Aim the plummet cross hairs to a target on the ceiling by turning footscrews on the bottom tribrach.

Rotate the top tribrach 180 degrees and view the target. If there is any misalignment, use the adjustment screws of the optical plummet to carefully adjust out half the error.

Rotate the top tribrach again by 180 degrees and repeat the process until there is no misalignment.